All insects, including mosquitoes, must be able to determine the palatability of potential food sources. This makes the use of sugar-baited traps an effective method to control the spread of mosquito borne illnesses. D. melanogaster is a model organism to study gustation because the fly can detect a broad range of compounds but has a relatively simple neurophysiology. Gustatory receptors (Gr) are expressed in taste neurons and members of this family are implicated in the detection of sweet or bitter tastants by genetic analyses. However, there has been little success in expressing these receptors in heterologous expression systems, which has led to a critical gap in understanding the functional properties of individual insect Grs, as well as in identifying the composition of a functional insect taste receptor. This represents an important problem because Grs belong to a family that is highly conserved in insects and non-insect arthropods. To address this problem, we will use two parallel approaches to study sweet taste receptors in Drosophila and mosquitoes, taking advantage of a novel in vivo expression system that we have designed for decoding Drosophila gustatory receptor proteins. Using this approach, I have identified sugar response profiles of each of the putative sweet taste receptors in D. melanogaster. Moreover, using recordings with mixed stimuli I have found that sweet taste receptors in Drosophila are directly inhibited by bitte alkaloids. This mechanism may be evolutionarily conserved since labellar sweet taste neurons in An. gambiae and Ae. aegypti are inhibited by alkaloids as well. The proposed project is innovative because this is the first report of successful expression of sweet taste receptors in a heterologous neuron in vivo. The hypothesis is that each receptor in both D. melanogaster and An. gambiae can individually respond to a unique set of sweet compounds and is inhibited by a distinct set of alkaloids. The rationale is that by understanding how insect taste receptors detect attractive and aversive compounds, we can then screen for novel compounds that can be used in insect control strategies. My objectives will be achieved via two specific aims: (1) Identify an characterize the functional properties of trehalose receptors in Drosophila, and (2) Identify and characterize the functional properties of select mosquito sweet taste receptors. This project will reveal the composition and response properties of a functional trehalose receptor. My studies will lay the foundation to probe structure-function relationships of sweet receptor's in Drosophila and An. gambiae in terms of detection of chemicals of both sweet and bitter categories with opposing effects on receptor activity. The significance of the proposed project is that it will advance our understanding of a highly conserved insect chemoreceptor family and establish a system to study insect gustatory receptor function by ectopic expression. The proposed project will enhance my training to be an independent researcher and allow me to use engineering principles to solve biological questions about sweet receptors. I will also gain experience in working with the disease vector An. gambiae.